This invention relates to a method of making a membrane which may be used in making a mask for use in lithography.
Different types of lithography are used in order to expose a photoresist which is deposited on the semiconductor wafer during the manufacture of integrated semiconductor circuits. Miniaturization and providing a large number of circuits on a chip requires the use of shorter wavelengths to obtain good resolution and small size. As a result, X-ray lithography involving short wavelengths which permit particularly fine features to be defined have been used.
Lithography, whether involving X-rays or other types of rays, generally utilizes a mask having a desired pattern thereon. The mask is interposed between a source of radiation and a semiconductor substrate coated with the resist on which a pattern is to be exposed. The mask must result in good definition of the lines being exposed, with the masked area being opaque to the radiation being used and the mask sustrate being transparent to that radiation.
The mask substrate which is transparent to radiation may be a thin film of the thermo-plastic polyester (polyethylene teraphthalate) or a thermoset polyimide sold under trademarks MYLAR and KAPTON, respectively. A typical material which may be used as the radiation absorber on the substrate may be gold.
Because of the relatively small size of the membrane used in making a mask for X-ray lithography, it is desirable to provide a carrier for handling the membrane until it is ready for additional tensioning and attachment by epoxy bonding to a mask support ring. It is important that the membrane is not damaged when it is removed from the substrate on which it is generally formed, which may happen if the substrate is dissolved by harsh chemical means.